The invention relates to a folding box, especially of oblong rectangular shape, having a rectangular, liquid-tight, sealed bottom formed by two pairs of confronting panels of the unslitted bottom portion of the box which are folded over at 90.degree. to the long axis of said box, two such confronting panels being plain panels and the other two forming each a triangular fold pocket which is laid beneath the plain panels, a narrow strip of the entire end margin of the box being folded together to form a fillet seam which is flattened against the folded sections.
In a folding box of this kind, the bottom is pressed in a die which is flat except for recesses of varying depth corresponding to the number of plies in the various areas of the bottom. With a die of this kind it is possible to apply a uniform pressure over the entire bottom area, which is necessary to prevent the folded bottom material from breaking along the creases.
A box which will be absolutely liquid-tight on the interior without pockets into which the liquid can penetrate can be produced in the above-described box when the bottom is pressed, simply by coating with a thermoplastic adhesive at least the inside of the end section of the box from which the bottom is formed. Before pressing, a box thus prepared is heated with hot air. In the die the thermoplastic adhesive cools, cementing together the superimposed areas of the bottom and sealing the seams.
It has been found to be disadvantageous that a box bottom which has been pressed in a flat die does not have very great stability because the bottom tends to bulge. Attempts to dish the bottom inwardly to create a circumferential edge to provide stability have encountered difficulty in avoiding leakage.